Bodolay et al. U.S. Pat. No. 3, 319, 538 shows a bag making machine comprising a continuous process line for making bags of plastic sheeting by heat sealing and cutting of a continuous length of plastic sheeting. The sheeting is folded longitudinally to define an intact, folded bottom edge. The bags may be filled with produce or the like as they are formed on the process line.
As a later step, the sides of the partially formed bag are simultaneously sealed and cut away, leaving a connected top strip of the plastic, to permit completion of the bag-forming process with a filling and a top-sealing step.
However, technical problems arise as the cutting takes place at or next to the hot, immediately-formed vertical seals. The seals can be damaged by the cutting blade since they are still fresh, hot, and soft, and plastic material can adhere to the blade since it is in viscous, liquid form. The plastic can adhere there, to interfere with the subsequent cutting process of the Bodolay et al. process line.
Furthermore, some bag containers are not suitable for the storage of onions, for example, which require a large degree of air ventilation in order to avoid premature spoiling, so that a bag made out of plastic sheeting on both sides is not suitable for use.
By this invention, a bag is provided which is suitable for the storage of onions and other products where a high degree of open ventilation is necessary or desired, while such a bag may be made on an automated process line from rolls of plastic material. Furthermore, a novel heat seal and cutting die is provided in which the cutting and the heat sealing may be simultaneous, but without the technical difficulties that arise in Bodolay et al. when heat seal lines are formed with a simultaneous cutting step.
By this invention, a process for the manufacture of bags made from plastic sheets is disclosed where, preferably, one side of the bag comprises a mesh sheet, which is typically made out of plastic, and which is heat sealed at its periphery to a solid-wall, thermoplastic sheet to form the complete bag. By the method of this invention, one advances a pair of sheets, one of which is a solid thermoplastic sheet and the other of which comprises a mesh, along a process line. One side of the thermoplastic, solid-wall sheet is folded over on a fold line to engage a corresponding side of the narrower mesh sheet. A longitudinal seal line is formed between them, which seal line is spaced from the fold line. One forms transverse slits in both of the sheets together, and one heat seals the sheets together at edges of the slits to form separate, sealed bag edges. The slits extend across edges of the combined sheets, but are spaced from the opposed, typically upper, edges.
One then drops produce or other desired materials between the open, upper edges of the two sheets. Thereafter, the bags may be heat sealed along the upper sheet edges thereof, preferably by sealing at a point spaced from the upper edges of the sheets of the process line, while cutting a line spaced from the upper edge seal, so that a waste strip of plastic sheeting is retained on the process line, and the bag, comprising a joined mesh sheet wall and solid plastic sheet wall, and sealed at all four sides, drops away.
Preferably, the sealing of the side edges and other edges of the bag may be accomplished by a heat seal die, which comprises first and second heat seal bars for joining the sheets with heat seal lines (both mesh and solid sheets). The bars are spaced from each other in parallel relation, but no more than about two inches apart and preferably less than one inch. A heated cutter edge for cutting aligned, linear cuts in the sheets (mesh and solid) is provided, with the cutter edge being positioned between the heat seal bars. A system is provided for heating the heat seal bars and the cutter edge, each above the softening temperature of at least one of the sheets and preferably both. An anvil is positioned to back the sheets, against which the heat sealing and cutting takes place.
The heat seal bars preferably carry a high-temperature stable anti-adhesive, such as one or more layers of polytetrafluoroethylene (PTFE), to prevent sticking of the plastic sheets to seal bars. Such sheeting may be made of material sold under the trademark Teflon.
Preferably a heater rod extends through a cutter body that defines cutter edge. A major portion of the heater rod is in contact with the cutter body, while a minor portion (typically on the order of about 10% or 20%) of said heater rod is in heat-flow contact with the heat seal bars. Accordingly, a greater heat flow passes to the cutter edge than passes to the seal bars, so that the temperature of the cutter edge is automatically greater than the temperature of the seal bars where they engage the plastic sheets for heat sealing. Thus, the cutter edge is highly effective to make a smooth cut through both sheets, as the seal bars form parallel heat seal lines spaced from the cutter edge.
Preferably, the heat seal bars comprise arms of an integral, U-shaped structure, so that they both may be heated from a single area of contact with the heater rod.
Preferably, the mesh sheeting is positioned so that one set of parallel strands of the mesh extends in the direction of motion of the sheeting along the process line, so that the mesh sheeting is longitudinally stretch-resistant.
The heat sealing and cutting die of this invention simultaneously provides a pair of spaced heat seals to form edges of separate bags, and a cut between the heat seals in the integral sheeting that forms the bag, with the cut being spaced from the respective seal lines so that the hot plastic of the seal lines is not disturbed by the cutting process. Thus, strong, peripheral seals are provided to a continuing series of bags as the manufacture thereof takes place.
A further improvement, which can be used with the heat seal die of this invention described above, or for other, conventional seal bars which are used as heat seal dies to form heat seal lines in thermoplastic materials, is disclosed below.
By this improvement, a heat seal die is provided for heat sealing plastic sheets, which die comprises a heatable heat seal bar having an outer heat sealing edge. A heat conductive metal strip member is carried on the outer heat sealing edge. The heat conductive metal strip member comprises: an elongated base portion and a plurality of spaced apart retention member portions extending in a common transverse direction from longitudinal edges of the base portion.
Inner surfaces of the base portion and the retention member portions define an elongated channel, having an open side opposite to the base portion. The channel grippingly receives the outer heat sealing edge, so that the metal strip member is carried on the heat seal bar, with the base portion of the metal strip member facing outwardly. The retention member portions may have some spring action, so that they may resiliently press against the heat seal bar for frictional retention on the heat seal bar, and they may comprise metal flanges, preferably with some cut spaces along their length and extending normally from the base portion. However, even a simple U channel metal strip member (of U-shaped cross section) may be used if it has suitable resilience.
The metal strip member carries a heat-stable plastic release layer at least on the outer face of the base portion. Thus, sticking of hot plastic to the metal strip member and the heat seal bar during heat sealing is suppressed.
As an added advantage, when the heat stable plastic layer on the base portion has worn out, another heat conductive metal strip member carrying a new plastic layer can be easily placed on the outer heat sealing edge of the heat seal bar. This can avoid an extensive, multiple hour operation in which the heat seal bar itself is replaced, in those circumstances when it is coated with a plastic coating, which is a conventional expedient, and that layer wears out. By the improvement described, the heat seal bar does not need replacement. Instead, an old, worn out heat conductive metal strip member is replaced with a new one, having a fresh, heat-stable plastic layer, for the avoidance of hot plastic sticking to the heat sealed die.
In some desirable embodiments, the retention member portions of the strip member may comprise a plurality of spaced fingers (short flanges) attached to the base portion, in a manner similar to the structure shown in Petri U.S. Pat. No. 4,769,966, which structure is taught to be used for a different purpose. A suitable type of heat conductive metal strip member may be purchased from Device Technologies, Inc. of Marlborough, Mass., which is normally sold for use as a grommet strip for non-molding uses. By this invention, the grommet strip may be encased in a tube of heat-stable plastic, for example, polytetrafluoroethylene, to provide a heat-stable plastic layer for particularly the outer face of the base portion of the grommet strip, which then may be used in accordance with this invention. However, any expedient for placing the plastic layer on the base portion may be used.
Preferably, the heat conductive metal strip member is made of a highly heat conductive metal such as aluminum.
The heat seal bar described above may, in accordance with this invention, be spaced in parallel relation from a second heat seal bar typically by no more than about 2 inches. The heat seal die utilizing the heat seal bar of this invention also may have a heated cutter edge for cutting linear cuts in plastic sheets at a position between the parallel, heat sealed bars. Furthermore, as described above, the heat seal bar may comprise an arm of a U-shaped structure (in cross section) comprising a pair of said heat seal bars.
Alternatively, a conventional, simple, single heat seal bar may also carry the heat conductive metal strip member of this invention having a heat-stable plastic layer, for the purpose of preventing hot plastic sticking during formation of heat seals.